The present invention relates to a hydraulic power system for supplying an operational fluid of a controlled flow rate and/or controlled pressure to a load line leading to a plurality of hydraulic actuators from a reservoir and vice versa.
U.S. Pat. No. 4,801,247 describes a variable displacement piston pump of the type in which its output flow and output pressure are electrically controlled by a proportional electro-hydraulic control valve. In this conventional hydraulic pump, the hydraulic pressure acting on a control piston and opposing to a spring force is controlled by the proportional electro-hydraulic control valve so as to adjust the tilt angle of a swash plate arranged inside the pump by the displacement of the control piston. In a flow control mode, the control valve is energized by an input current corresponding to the difference between a flow command signal and a flow detection signal to communicate the pressure chamber of the control piston with a pump discharge port or a tank line with an opening proportional to the input current. When the output pressure reaches a certain predetermined pressure value, the control mode is changed to a pressure control mode and the tilt angle of the swash plate is controlled in the vicinity of a cutoff position. This conventional control system relied on the tit angle of the swash plate is advantageous in that both the flow control and the pressure control are effected by the single proportional electro-hydraulic control valve thereby smoothly effecting changeover between the flow control mode and the pressure control mode. In this conventional hydraulic pump, however, the hydraulic control system for controlling the tit angle of the swash plate is complicated in construction so that the flow control characteristics in the low pressure region are deteriorated and the driving motor must always be rotated during the operation of the pump irrespective of the presence or absence of output flow thus giving rise to a problem that the pump is disadvantageous from the standpoint of energy loss.
Japanese Laid-Open Patent Application No. 10-131865 describes a hydraulic pump of the type in which its rotational speed is controlled by a servomotor to supply a hydraulic fluid of a flow rate corresponding to the rotational speed. The servomotor is controlled by an inverter in accordance with the difference between a speed command signal and a rotational speed detection signal. The pump output pressure is detected by a pressure detector and also the difference between a pressure command signal and a pressure detection signal is detected. The detected pressure difference signal is combined with a non-proportional function signal produced according to the rotational speed detection signal so that a smaller one of the resulting sum value and a flow command value is used as a speed command signal. This pump control system relied on the rotational speed is disadvantageous in that there is a limitation to the changeover stability and continuity due to the addition of a non-proportional function signal to a pressure command signal for the purpose of changeover between the flow control and pressure control modes for the rotational speed control loop of the servomotor and that this control system is inferior in response characteristic to the swash plate tilt angle control system due to the driving of the servomotor by the inverter control.
It is the primary object of the present invention to provide a hydraulic power system capable of realizing control characteristics which are more excellent than those of the previously mentioned prior art.
It is another object of the present invention to provide a hydraulic power system capable of realizing a stable and smooth changeover between control modes and higher response characteristics by the use of a hydraulic pump which is relatively simple in construction and easy in maintenance.
According to a preferred embodiment of the present invention, the foregoing objects can be accomplished by a hydraulic power system for supplying hydraulic fluid of a controlled flow rate and/or controlled pressure from a reservoir to a load line communicating with a hydraulic actuator or vice versa, said system comprising:
a reversible hydraulic pump motor having a rotary shaft and adapted to serve as a hydraulic pump for feeding the fluid at a flow rate corresponding to the rotational speed of the rotary shaft rotating in a forward direction when feeding the fluid to the load line from the reservoir and to serve as a hydraulic motor for rotating the rotary shaft in a reverse direction at a rotational speed corresponding to the flow rate of the fluid when feeding the fluid to the reservoir from the load line,
a variable-speed servomotor having a driving shaft coupled to the rotary shaft in a torque transmitting manner, the driving shaft being rotatable in either of the forward direction and the reverse direction with both a rotational speed and a rotation direction corresponding to the driving current supplied to the servomotor,
pressure detecting means for generating a first electric signal corresponding to the fluid pressure in the load line,
rotational speed detecting means for generating a second electric signal corresponding to the rotational speed of the driving shaft,
signal command means for generating a preprogrammed pressure signal and a preprogrammed flow command signal,
signal processing means for preferentially generating a speed command signal of a magnitude corresponding to the flow command signal by a limiter operation when a pressure difference signal corresponding to the difference between the pressure command signal and the first signal is higher than a predetermined limiting level and for preferentially generating a speed command signal of a magnitude corresponding to the pressure difference signal when the pressure difference signal is equal to or lower than the limiting level, and
rotational speed control means responsive to the speed command signal and the second signal for controlling the magnitude of the driving current supplied to the servomotor through a closed feedback loop of the rotational speed so that the rotational speed of the servomotor corresponds to the speed command signal.
The hydraulic power system according to the present invention is usable in applications in which a hydraulic power is supplied to a machinery including a hydraulic actuator requiring a continuous and smooth changeover between the respective modes of flow control and pressure control, e.g., an injection molding machine, hydraulic press machine, hydraulic press fitting machinery, hydraulic bending machine and the like. In these machines, an energy is inputted in the form of an electric energy which is converted to a hydraulic power by the hydraulic power system of the present invention and this hydraulic power is substantially coincident with the desired values of a flow rate and a pressure to be supplied to the actuator in the respective operating phases of the machine. Thus, by using the hydraulic power system of the present invention, there is no need to use the electro-hydraulic proportional control valve required for controlling these desired values in the conventional systems.
The hydraulic power system according to the present invention employs the reversible hydraulic pump motor which functions as a hydraulic pump during its forward rotation and which functions as a hydraulic motor during its reverse rotation. While a variable displacement type can be used for this hydraulic pump motor, preferably use is made of a fixed displacement type which is relatively simple in pump construction and easy in maintenance. The variable-speed servomotor having the driving shaft connected to the rotary shaft of the hydraulic pump motor in a torque transmitting manner is also a reversible type and preferably a magnet-field synchronous AC servomotor can be used for it.
In the hydraulic power system according to the present invention, control commands are respectively supplied in the form of a pressure command signal and flow command signal which are preprogrammed according to the respective operating sequences of the hydraulic actuator from the signal command means that can be composed for example of a programmable controller or computer. The detector means for the controlled variables are composed of pressure detecting means for generating a first electric signal corresponding to the fluid pressure in the load line communicating with the hydraulic actuator and rotational speed detecting means for generating a second electric signal corresponding to the rotational speed of the driving shaft of the servomotor. Preferably a semiconductor pressure transducer can be used for the pressure detecting means and preferably a rotary encoder can be used for the rotational speed detecting means.
In the hydraulic power system according to the present invention, the control system of the servomotor includes in combination a first feedback loop forming a minor loop for rotational speed control system with a negative feedback signal corresponding to the rotational speed of the hydraulic pump motor and a second feedback loop forming an outer loop of the minor loop for pressure control system with a negative feedback signal corresponding to the fluid pressure in the load line. The principal parts of the rotational speed control system are included in the rotational speed control means and the principal parts of the pressure control system are included in the signal processing means.
According to another advantageous embodiment of the present invention, the rotational speed control means includes means for generating a rotational speed difference signal corresponding to the difference between the speed command signal and the second signal, torque limiter means for limiting the upper and lower limits of the rotational speed difference signal to come within a predetermined range, and current control means for receiving the rotational speed difference signal limited by the torque limiter means as a control input and the second signal as a feedback signal to feedback control the magnitude of the driving current to be supplied to the servomotor.
According to still another advantageous embodiment of the present invention, the signal processing means includes difference signal detecting means for receiving the pressure command signal at its positive input and the first signal at its negative input to generate a pressure difference signal corresponding to the difference between the two input signals, signal limiter means for producing an output signal of a magnitude corresponding to the pressure difference signal when the pressure difference signal is equal to or less than the limiting level and for holding the magnitude of said output signal at a given level when the pressure difference signal is higher than the limiting level, and means for generating a signal corresponding to the sum or the product of the flow command signal and the output signal of the signal limiter means as the speed command signal.
In a condition where the hydraulic fluid pressure in the load line is less than the pressure command, the controlled deviation or the pressure difference signal corresponding to the difference between the first signal generated from the pressure detecting means and the pressure command signal has a high value exceeding the limiting level. The signal processing means gives preference to the flow command by a limiter operation so that the speed command signal varying in response to the flow command signal is applied to the rotational speed control means. In this condition, the pressure difference signal is held at a fixed value equal to the limiting level by the limiter operation so that the control command for the control system of the servomotor is governed by the flow command signal and the rotational speed control means controls the rotational speed of the servomotor in such a manner that the flow command signal and the feedback signal from the rotational speed detecting means substantially coincide with each other. This condition is the flow control mode.
Here, in accordance with the present invention the term xe2x80x9cpreferentialxe2x80x9d means that in the flow control mode the flow command signal becomes predominant in the speed command signal applied to the rotational speed control means, that is, the speed command signal includes the flow command signal as a control command and the pressure difference signal of a fixed value equal to the limiting level.
On the other hand, when the fluid pressure in the load line reaches the pressure command, the controlled deviation or the pressure difference signal corresponding to the difference between the first signal generated from the pressure detecting means and the pressure command signal assumes a low value of less than the limiting level so that the signal processing means gives preference to the pressure difference signal and the speed command signal varying in response to the pressure difference signal is applied to the rotational speed control means. In this condition, the control command for the control system of the servomotor is governed by the pressure difference signal and the control system forms a pressure control feedback loop including in series the rotational speed control system as a minor loop. Thus, the continuity is ensured for the speed command between this pressure control mode and the flow control mode and the rotational speed control means controls the rotational speed of the servomotor in such a manner that the pressure difference signal and the feedback signal from the rotational speed detecting means substantially coincide with each other. This condition is the pressure control mode.
In accordance with the present invention, the foregoing mode changeover operation, that is, the operation of changing the speed command signal from the flow command signal to the pressure difference signal or vice versa is effected continuously and smoothly owing to the fact that the continuity of the speed command is maintained between the two modes by the limiter operation as mentioned previously, that also in the flow control mode the pressure difference signal is included with a fixed value equal to the limiting level in the speed command signal, and that the pressure difference signal varies between the limiting level and a lower level without overshooting during the bidirectional transfer between the flow control mode and the pressure control mode. To achieve this changeover operation by the selection operation or the switching operation as in the conventional manner is not preferable since there is the possibility that the changeover between the control modes becomes discontinuous. It is to be noted that to additionally provide means for causing the pressure difference signal to follow up the flow command when the control system is in the flow control mode is preferable from the standpoint of more smoothly effecting the changeover between the modes without any shock.
With the hydraulic power system according to the present invention, the control of the rotational speed of the servomotor can be effected for both the forward rotation and the reverse rotation so that the control can be effected electronically in both cases where the hydraulic pump motor is rotated in the forward direction so as to feed the hydraulic fluid of the desired flow rate to the load line and also to control its pressure at the desired value and where the hydraulic pump motor is rotated in the reverse direction so as to reduce the fluid pressure in the load line according to the optimum pressure reducing speed pattern. It is of course possible to control the hydraulic pump motor at an extremely low rotational speed or in its substantially halting state and thus the control is stable even in the low pressure region of the load pressure.
With the hydraulic power system according to the present invention, in the pressure control mode the fluid pressure in the load line is detected by the pressure detecting means and the resulting pressure detection signal acts effectively as a feedback signal on the closed control system. As a result, the fluid pressure is subjected to the closed control even if the temperature of the fluid changes from the normal operating temperature, thereby automatically making the fluid temperature compensation effective. On the other hand, in the flow control mode the fluid pressure is subjected to the open-loop control so that when there occurs any change in the pressure due to a fluid temperature change, it appears as a change of the flow rate for the hydraulic actuator.
In accordance with still another advantageous embodiment of the present invention, the system further includes fluid temperature detecting means for detecting the temperature of the hydraulic fluid to produce a third electric signal of a magnitude corresponding thereto, and temperature compensation means for applying to the flow command signal or the speed command signal a correction amount equivalent to a variation of the temperature detected by the fluid temperature detecting means with respect to a predetermined reference temperature.
The fluid temperature detecting means can be arranged at an arbitrary position in the hydraulic circuitry including the hydraulic fluid reservoir, the hydraulic pump motor, the load line and the hydraulic actuators. The fluid temperature compensation is effected by detecting the difference (variation) between the fluid temperature detected by the fluid temperature detecting means and the reference temperature (presettable to any given temperature) and adding to the flow command signal a signal correction amount equivalent to a flow variation (determined by the characteristics of the fluid used) corresponding to the detected variation. It is to be noted that this signal correction amount may be added to the speed command signal and in this case the fluid temperature compensation is made effective not only in the flow control mode but also in the pressure control mode.
In accordance with still another advantageous embodiment of the present invention, the system further includes correction means for correcting the flow command signal with the first signal to compensate a variation in the pump volumetric efficiency. This correction means can be constituted by a differential operational amplifier which receives the flow command signal at its positive input terminal and the first signal from the pressure detecting means at its negative input terminal with a suitable correction factor. In the flow control mode, as for example, a compensation is provided for a drop in the pump volumetric efficiency due to such cause as an increased leakage flow within the pump due to an increase in the load pressure.
In accordance with still another advantageous embodiment of the present invention, the hydraulic power system further includes operating speed detecting means for producing a fourth electric signal corresponding to the operating speed of the hydraulic actuator, operation discrimination means responsive to the fourth signal to discriminate whether the actuator is in operation, and operating speed control means for additionally feeding the fourth signal back to the speed command signal and subjecting the operating speed of the actuator to the closed control only when the operation of the actuator is discriminated by the operation discrimination means.
As in the case of an injection molding machine in which the operation of a plurality of hydraulic actuators is controlled by a single hydraulic pump, for example, operating speed detecting means is arranged for the purpose of detecting the operating speed of at least particular one of the hydraulic actuators which requires a particularly highly accurate speed control in addition to the compensations for working fluid temperature variations and load pressure variations. Such operating speed detecting means can be arranged for each of the plurality of hydraulic actuators provided that in this case the operations of the respective hydraulic actuators do not overlap each other in time.
In the case of the injection molding machine, for example, the hydraulic actuator which requires the highest degree of accuracy for the operating speed control is the injection cylinder and therefore the operating speed detecting means is mounted to this injection cylinder. When the actuator having the operating speed detecting means mounted thereto (i.e., the injection cylinder) comes into operation, the operating speed detecting means produces a fourth signal. When the fourth signal is prodiced, the operation discrimination means discriminates that the particular actuator is in operation, whereas it is determined that the particular actuator is in the non-operating condition if there is no generation of the fourth signal. The operating speed control means additionally feeds the fourth signal back to the speed command signal to subject the operating speed of the particular actuator to a closed control only when the operation of the particular actuator is discriminated by the operation discrimination means. This control is effective irrespective of whether the system is in the flow control mode or in the pressure control mode.